The present invention relates to a semiconductive photodetector device having a construction in which a great number of photodiodes are juxtaposed on one major surface of a semiconductor substrate.
As well known in the art, intensity of light irradiated on a pn junction can be measured by reading voltage or current representative of an amount of carriers created by the irradiation of light. The semiconductive photodetector device based on this phenomenon has been used as a photosensor unit in an imaging apparatus, an OCR (optical card reader), a position detecting apparatus, a spectrophotometer or the like since its light receiving portion can be miniaturized by using integrated circuit technique to thereby obtain high resolution and it can be made integral with its peripheral circuits such as a detection signal processing circuit and a drive circuit to thereby assure high reliability. In this type of semiconductive photodetector device, a great number of photodiodes are juxtaposed at predetermined intervals on one major surface of a semiconductor substrate, on which major surface the detection signal processing circuit, drive circuit and the like circuit are formed by integrated circuit technique. The semiconductive photodetector device comprised of the juxtaposition of the great number of photodiodes is required to provide high resolution and to have a wide dynamic range within which output signals varies with the intensity of incident light.
High resolution can be accomplished by increasing the number of photodiodes to be integrated on the semiconductor substrate. With a view to improving yield, on the other hand, desirability is such that the semiconductor substrate be reduced in size as small as possible. Accordingly, in order to improve the resolution, the junction area of the photodiode must be minimized. With a decreased junction area of the photodiode, however, the dynamic range will disadvantageously be narrowed as will be described later.
Meanwhile, as far as the dynamic range is concerned, the lower limit can be lowered by decreasing the dark current and the upper limit can be raised by increasing the saturating exposure amount.
The dark current is mainly due, for one thing, to deterioration by light irradiation, particularly of ultraviolet light, of a portion of the pn junction exposed to the semiconductor substrate surface and, for another thing, to crystalline properties present in the vicinity of the pn junction. The former cause can be suppressed by providing a light shielding film above the exposed portion of pn junction to optically shield the same and the latter cause can be suppressed by minimizing the junction area. However, reduction in the junction area decreases the saturating exposure light amount as will be described later, leading to a disadvantage that the upper limit of the dynamic range is lowered. For the purpose of shielding, a light shielding film 105 is usually formed as shown in FIG. 1. Even with the light shielding film, the irradiated light is subject to multiple reflections as illustrated by dotted lines and penetrates into a portion underlying the light shielding film 105 with the result that deterioration (i.e. experimentally observed increase of dark current across the pn junction) still takes place at an edge of a pn junction 106. This disadvantage will be aggravated as a photo-insensitive region between adjacent photodiodes is made narrower so as to improve the resolution. For example, employing the 5 .mu.m rule LSI technique, the photo-insensitive region has a width of 46 .mu.m whereas employing the 3 .mu.m rule LSI technique, width is reduced at a ratio of 1/2.4, measuring 19 .mu.m. Correspondingly, the width of the light shielding film is also narrowed and hence, the penetration of light by way of the multiple reflections is increased. In FIG. 1, reference numeral 101 designates a semiconductor substrate of one conductivity type, 102 semiconductor regions of the other conductivity type formed in one major surface 108 of the semiconductor substrate 101 in juxtaposition relationship, 103 an insulating film formed on the one major surface 108, and 104 electrodes making ohmic contact to the semiconductor regions 102. In an integrated circuit, the insulating film 103 is comprised of a silicon oxide film 1031 in close contact with the surface of semiconductor substrate 101 and a layer 1032 of phosphosilicate glass formed on the film 1031. A major portion of the electrode 104 and the light shielding film 105 are surrounded by the phosphosilicate glass layer, so that oxidization of the electrode 104 and light shielding film 105, both made of aluminum, can be prevented. Reference is made to Proceedings of the Custom Integrated Circuits Conference, IEEE, May 1982, pp. 161-165.
The saturating exposure amount will now be explained. When the photodiode is irradiated with light of high intensity or long duration, a phenomenon occurs wherein the photodiode is saturated during a period for reading signals, that is to say, electric charges stored in the photodiode at the beginning of the signal reading period are recombined with hole-electron pairs created by the light irradiation to disappear before the signal reading period terminates. When the photodiode is saturated, output signals proportional to intensity of incident light can not be produced. Further, when the irradiation of light on the saturated photodiode continues, electric charges excited by photons overflow, turning into noises which have an adverse effect on the other photodiodes. Thus, the saturating exposure amount is defined to denote resistance against the saturation of photodiode. The saturating exposure amount can be increased by increasing the amount of carriers to be stored at the beginning of the signal reading period, namely, increasing the junction capacitance. In order to increase the junction capacitance, the area of pn junction must be increased. However, increase in the pn junction area disadvantageously results in increase in the dark current and decrease in the resolution.